1. Field of the Invention
The present invention relates to an image reading unit and an overhead image reading device.
2. Description of the Related Art
There have been various types of image reading devices which read an image of a document and electrically performs a processing, and an overhead image reading device having improved facility of reading which is attained by reading an image by placing a document on a placement table or the like and imaging the document from above a reading surface of the document is included in the image reading devices.
For example, in the imaging device disclosed in Japanese Patent No. 2860119, a pedestal which has a bendable support column provided upright is placed on a placement surface on which an imaging object is placed, and the support column provided upright on the pedestal is provided with a camera for imaging the imaging object. With such configuration, it is possible to image the imaging object placed on the pedestal from above with the camera, and it is possible to change an imaging range by bending the support column during the imaging.
In the image scanner device disclosed in Japanese Patent No. 2982614, an image reading unit having an illumination device, an image sensor, a lens, and a reflection mirror is held in such a manner that the image reading unit is attached to a document placement table by using a stand arm for movably holding the image reading unit. With such configuration, it is possible to read an image of the document placed on the placement table from above the document by the image reading unit, and it is possible to enlarge a space on the document placement table by moving the image reading unit when the image reading unit is not used.
In the image scanner disclosed in Japanese Patent No. 3027915, a reading unit provided with a one-dimensional image sensor, a lens, a reflection mirror, and a two-dimensional image sensor is held by an arm attached to a placement table and further has a display device for displaying an image which is read at least by the two-dimensional image sensor. With such configuration, it is possible to read the image more appropriately by reading by the two-dimensional image sensor, reading the image of a document from above the document by the one-dimensional image sensor based on the image displayed by the display device.
Japanese Patent No. 3931107 discloses the non-contact image reading device having a support column for supporting a camera for reading an image and a placement table which is movable in a horizontal direction. With such configuration, it is possible to obtain an image of a large size document by imaging every predetermined ranges of the document from above with the camera by moving the placement table and combining the plurality of obtained images.
In the case of reading an image of a document by the image reading device described above, the image is read by receiving light reflected from the document. However, in the case where a light source for illuminating the document is not provided as in the image scanners or the like disclosed in Japanese Patent No. 3027915 and Japanese Patent No. 3931107, the imaging is performed by utilizing environmental light which is ambient light of the device. Image quality depends on environmental light in the case of imaging a document by the environmental light, but the environmental light is subject to large change depending on a place in which the image reading device is installed and a state of reading. Therefore, the image quality is unstable in some cases when the imaging is performed by utilizing the environmental light.
In contrast, in the case where the light source for illuminating the entire document is provided as in the imaging devices disclosed in Japanese Patent No. 2860119 and Japanese Patent No. 2982614, it is possible to ensure a predetermined luminance, but the irregularity in luminance depending on a portion of the document can occur when the entire document is irradiated with irradiation light from the light source. In this case, since irregularity occurs in the reflection light reflected from the document, an image read by receiving the reflection light sometimes has remaining unstable image quality though it is possible to ensure brightness of an entire portion of the read image. Further, in the case where the entire document is irradiated with the irradiation light from the light source as in the imaging devices disclosed in Japanese Patent No. 2860119 and Japanese Patent No. 2982614, it is necessary to irradiate the light in a wide range. Therefore, it is necessary to increase a quantity of the light from the light source, and there has been a tendency that power consumption by the light source is increased.